It is well known in the prior art that semi-conductor light sensors cannot respond instantaneously to abrupt changes in light intensity. This effect includes both buildup and decay lags, where buildup lag is caused by the inability of the photosensor to fully respond to light at the very instant that the light is first applied and decay lag is caused by the inability of the photosensor to fully decrease the output signal level to its dark value at the instant that the light is removed. An article, "Imaging Devices Using the Charge-Coupled Concept," by David F. Barbe, published in the Proceedings of the IEEE, vol. 63, Jan. 1975, details the causes of the lag effect.
A well-known solution to the problem of lag is to provide the semi-conductor light sensor with a bias light or "fat zero" in addition to the incident illumination. The use of a bias light and its effects on the operation of the light sensors is discussed in a number of articles, among them, "An Application Note: Photosite Transfer Loss (PSTL)," published in the 1989 Fairchild Weston CCD Imaging Databook, by the Fairchild Weston Systems, CCD Imaging Division, and "Video requirements for digial subraction angiography," by Horace H. Hines, et al, published in Medical Imaging and Instrumentation '84, SPIE vol 486, April 1984.
Apparatus for reducing lag utilizing a white light lamp is disclosed in the article, "Features of a New Three-Tube Color Television Camera," by D. G. Perkins and published by the Journal of the SMPTE, vol. 81, Jan. 1972. The apparatus comprises a "diascope" placed between the camera lens and the RGB color component pickup tubes. The diascope is built of prisms which project the bias white light from its source to each of the pickup tubes.
S. B. Mende, in an article, "A Low Light Level Slow Scan TV Camera for Satellite Application," discloses a method for bias illumination consisting of mounting light sources on the camera tube to relatively uniformly illuminate the photocathode surface.
G.B. Patent 1,240,468 discloses a bias illumination apparatus comprising a camera lens, a beam splitting system to split the incoming white light into its component RGB colors, and a fiber optic cable which transmits white light from a source to a location between the camera lens and the beam splitting system for the purpose of bias illumination.
In an article "One-inch diameter Plumbicon.TM. camera tube type 19XQ," by J. Dolledamp, Mulliard Technical Communications, No. 109, Jan. 1971, two methods are disclosed for reducing the lag. One is the incorporation of an electron gun with a high speed readout gun and the second is the incorporation of bias illumination via a lightpipe.
The abovementioned articles and patents have illustrated front illumination (i.e. including the bias illumination with the incoming light signal). It is also known to back illuminate, or to illuminate the sensor from a direction which is not part of the optical imaging path. For cathode-ray tubes, the back illumination is achieved by illuminating the internal side of the tube face. CCD sensors have no optical access to the back of the sensitive layer and thus, back illumination is not utilized with them.
Additionally, the abovementioned articles and patents have utilized white light sources which are typically incandescent sources. Incandescent sources have the disadvantage that the color balance achieved changes with changing intensity. Other light sources, such as xenon lamps, are complicated to operate, are relatively unstable, have a short life and are expensive to buy. Flourescent lamps, yet another choice for a white light source, are large, a ballast is required and the control of the intensity is problematic.